The present invention relates to a polymer gel barrier for the containment of hazardous waste in the soil.
Soil and ground water contamination has become a more and more pressing issue due to growing public concern about environment and health. Remediation of soil contamination can be achieved by removal of the contaminated soil, by chemical or biological modification at the contamination site, and by retention of the contaminants within the soil in order to prevent migration to aquifers.
The removal of contaminated soil is expensive due to the use of heavy machinery and the required transport to a designated storage or remediation location. Furthermore, the removal of contaminated soil without further chemical or biological treatment does not solve the contamination problem, but merely moves the problem to a different site.
Chemical and/or biological treatment of the contamination at the site is attractive, but depends greatly on the nature of the contaminants and the soil chemistry.
Chemical treatment generally is based on the fixation of the contaminants within the soil in order to prevent further migration into the aquifer. Such an approach is usually only practical for small and well defined sites and for very specific contaminants.
Biological treatment of contamination within the soil takes advantage of indigenous microorganisms at the site or employs specially engineered microorganisms designed to degrade a certain contaminant or class of contaminants.
Retention of contamination within the soil is usually a temporary measure and relies on establishing a barrier to drastically reduce contaminant transport and diffusion. Retention barriers are only feasible if horizontal strata that are impermeable to the contaminants are present within the soil at a relatively shallow depth so that a vertical barrier can be constructed down to the impermeable stratum at reasonable cost. If no horizontal, impermeable strata are present, it will be necessary to provide an engineered horizontal barrier. This is possible from a technological standpoint, but proves to be very expensive.
The use of grouting is a temporary measure for dispersal retardation of contaminants during the period of removal of the contaminants by other means. For example, a vertical wall can be constructed by injection of a cement-bentonite grouting material above an impermeable horizontal clay layer so that contaminant migration through the wall is reduced to a minimal amount over a period of years.
However, the injection of cement-bentonite-based grouting material is technologically difficult because great pressures are required for injecting the cement-bentonite slurry into the ground. Also, it is difficult to control the extent of the injection range.
Cement-based materials for engineering a retention barrier are the most commonly used materials despite the high cost of pumping for injecting the materials into the ground. Such materials have the advantage that they set hard and are substantially water-resistant. By adding adsorbing materials such as bentonite an enhanced surface area is provided and the cation exchange capabilities are increased. Such materials have the ability to bind many organic or inorganic compounds: for example, metals are immobilized by the formation of hydroxides or basic carbonates. One drawback is that cement mixtures have a high pH (greater than 12) and certain toxic metals, such as lead, chromium, copper, and nickel, show an increased solubility at such high pH values. Thus, such a cement barrier potentially could enhance the transport of such metals. Another disadvantage is that the setting of cements can be retarded by organic solvents and oils, by sulfates, and halides.
A general technical problem of conventional barrier construction by grouting or by slurry wall techniques is that excavation or pumping or both must be employed in order to put the viscous substances in place. Such grouting or slurry wall techniques are thus limited in their application because of their excessive costs. Another disadvantage is that their application is limited to shallow depths.
Lime-based (calcium oxide or hydroxide) barriers are very similar in their behavior to the barriers made of cement-based material. These materials behave similarly to the cement-based materials and are also characterized by a high pH value and slow setting properties. Water glass polymers (alkali silicates) have also been used as retention barriers. For example, the Chemfix process employs water glass. Such water glass barriers work well for transition metal contaminants since insoluble silicates are formed. However, silicate barriers are ineffective for the retention of chloride ions and monovalent cations.
It has also been suggested to use thermoplastic barriers as a retention means for contaminants. However, thermoplastic materials require heating of the material for injection and thus make the injection process very expensive. Furthermore, thermoplastic materials upon heating release organic components. Also, oxidizers, sulfates, or halides can undergo undesirable reactions with these thermoplastic materials.
It is also known to use organic polymer barriers based on urea-formaldehyde resin and polyacrylamide. Polyacrylamide has been employed by the oil industry for tertiary recovery processes where advantage is taken of the high viscosity and immiscibility of these polymers with oil and water. However, some organic solvents and oils may retard gelation of the polymer material and reduce the efficiency of the barrier.
It is an object of the present invention to provide a method for forming a containment barrier with a polymeric barrier material that is soluble in water before gelation and insoluble after gelation. The polymeric barrier material should have a controllable gelation time and a viscosity before injection similar to that of water in order to substantially reduce pumping costs.